Chess, it's a classic right? So how about a nice game of Chess? I am in fact talking about Lichess, a free and open source browser-based online Chess game.
You might think it funny but until recently, I had somehow never heard of it. It's been around since 2010, starting off a simple hobby project and it has since grown into one of the most popular Chess websites around. Ten years later it's still kept the original promises of remaining free and open source and it just seems to have continually grown. If you believe Alexa rankings, it's in the top 2,000.
With close to 80K players online at the current time of writing, it's easy to see it's popular and it has every right to be. It works perfectly in any browser I tried (including mobile!), it has no adverts, it doesn't require an account, it's totally open source (even the Android build is open source), it has a ton of puzzles, it can teach you to play and so much more. Lichess is not just a browser-based Chess game, it's a huge assortment of different modes and a big community to back it up. Not only all that, you can also sit and watch games too — if that's your thing.
If, like me, you had no idea it was a thing and apparently you also live under a rock, I hope this was helpful for you.
You can play for free online, against friends and a lot more on Lichess.org.